Review of egusphere-2022-1495

I consider this a very relevant paper. It is well written and reports on extensive field observations in the Saigon tidal river, which I consider very valuable. Also the conclusion that the plastic travels faster than the fresh water component of the tidal flow is relevant. But the calculation of the tidal volumes and efficiencies is, unfortunately, doubtful because of erroneous or unclear mathematics. So, until this unclarity is solved, we cannot judge if this conclusion is correct.

In line 155, the authors mention that ebb flow is positive and flood flow is negative. Fair enough. But in that case, the integral of the tidal volumes, V_ebb and V_flood, should also be positive and negative.

This makes the calculation of the delivery ratio unclear. This calculation should then be:

d_r= Delta(V)/V_tidal or

d_r= (V_ebb+V_flood)/(V_tidal)

V_tidal should be the average of the ebb and flood volumes: (Vebb-Vflood)/2.

If, instead the absolute volumes of the ebb and flood flow are taken, then the equation becomes:

d_r= 2*(V_ebb-V_flood)/(V_ebb+V_flood)

This would be:

In the case of plastic transport: the delivery of plastic to downstream in relation to the tidal transport of plastic.

In the case of river flow: the delivery of fresh water over the tidal volume or (Q_fresh*T)/(Tidal volume), which is approximately equal to the ratio between the fresh water velocity and the average ebb or flood tidal velocity.

One would expect these two delivery ratios to be the same, but interestingly they are not.

Now three things are wrong in Eq.(6)

First, the negative sign in the numerator and the positive sign in the denominator, unless absolute values of the volumes are implied, but then Eq.(9) would be wrong; it should then have a minus in the numerator.

Second the factor 2. There should be a factor 2 in the numerator of Eq.(6)

Third the unnecessary (and silly) addition of 100% in Eq.(6). As the authors should be aware, 100%=1, so there is no need to multiply by 100%. If a delivery value is 18%, then that is the same as 0.18.

Fortunately, it seems as if the equations are wrong, but the calculations are right. A quick look at the calculations in Table 1 suggests that they are correct.

2*3.1/(15+8.6)=0.27

2*170/(1100+790)=0.18

The delivery ratio of the flow velocity is calculated as 0.18. This seems to me a realistic value for the ratio between the freshwater volume and the tidal volume, so it seems as if the calculations are right. In that case the equations provided are wrong and the calculations are correct. But without more detailed insight I cannot conclude either way.

Then the interesting question remains why the plastic has a higher delivery ratio than the fresh water. My hypothesis would be that this has to do with the lateral distribution of the floating plastic. Floating plastic has the tendency to concentrate mid-stream, particularly during ebb (from my own observations in many tidal rivers). In midstream the surface flow velocities are largest. The concentration of floating objects (also water hyacinth) in mid-stream is due to the helix movement of water in river bends where floating objects are brought together. At slack time the water slacks earlier near the banks which causes a lateral movement of the surface water towards the banks. At flood flow, the floating objects are spread over the width and may even be trapped in the banks. The net transport of floating objects is then less. Maybe you can check if your observations confirm this. In any case, the data suggest that the plastic is discharged faster to the ocean than the average (freshwater) flow velocity suggests.

In line 310 the authors observe that the travel times of plastic in estuaries is long. This is not surprising since estuaries have an exponential shape (see Savenije, 2012) with cross-sectional areas orders of magnitude larger than the river cross-section. The section of observation in the Saigon River is rather far from the ocean, so the increase of the cross-sectional area is still modest compared to the river width, but further down the widening is much more and hence the delivery ratio will reduce substantially as one moves downstream. Generally, in alluvial estuaries the net downstream velocity is much smaller than that of the river feeding the estuary. So, retention times in estuaries are long and there is nothing special about it. But what is surprising is that the delivery ratio of the plastic is higher than the net freshwater transport. That is an interesting finding!

In Section 4.3 the authors discuss this issue. I think the explanation of entrapment (particles getting temporarily stuck during the flood flow) is realistic, but I would like the authors to ponder on the lateral distribution of the floating plastic. The fact that the floating plastics have a higher delivery ratio than submerged plastic supports my idea of the lateral distribution playing a role. I guess that your observations at the bridge with 5 observation sections could be used to investigate this hypothesis.

In lines 446-447 the authors suggest that it is the tidal movement that hampers the transport of the plastic. This is not true. It is the exponential shape of the estuary that enhances travel times. As one moves downstream, the cross-sectional area increases exponentially and the delivery ratio will decrease proportionally, because the tidal volume is directly proportional to the cross-sectional area (see Savenije, 2012).

Minor observations:

Line 200. The “total river surface length” (the integral of the tidal velocity) is called the “tidal excursion”. The ebb excursion is larger than the flood excursion, the difference being the integral of the freshwater velocity. As one moves further downstream the difference between these two excursions becomes smaller until, near the estuary mouth, the difference can hardly be observed anymore.

Reference:

Savenije, H.H.G., 2005, 2012. Salinity and Tides in Alluvial Estuaries, Elsevier. Completely revised 2nd edition in 2012, available from www.salinityandtides.org.

This paper presents observations of plastic transport in relation to flow velocity and discharge in the tidal part of the Saigon River. It is an interesting, well-written paper with novel observations of plastic transport. I will refrain from the interesting points that Reviewer 1 mentioned and the follow-up response by the authors and focus more on the methods of this research. Regarding the monitoring of plastics, in line 115 and in Section 2.3 (and in other parts) you mention that the counting of the plastic particles was done visually. How exactly? From the top of the bridge that is 14 m above the water or the plastics were somehow collected and sampled? While reading, in the beginning I assumed the former (which would then lead to the obvious question how accurate this data monitoring is when observing plastics as small as 0.5 cm) but when you mentioned the mass of the plastics in Equation (2) I assumed that you collected the plastics to weigh them. Then, based on the lines 231-235, did you actually collect and classify the plastic samples or did you visually observe them from the bridge and used the distributions from van Emmerik et al. (2019)? This part (and the Section 3.3) is very confusing, please clarify how you sampled the plastics and what is the role of the data from van Emmerik et al. (2019). This part is also critical for the interpretation of Figure 4 and for the analysis related to the different categories of plastics.

In my opinion, for an experimental study there are many assumptions when processing the data and many speculations when interpreting the results, which make the conclusions a bit doubtful. One of the key findings of the paper, according to the authors, is that the net transport of plastics is higher than the net water discharge. However, the water discharge was estimated based solely on near-surface velocity measurements in a flow environment that can potentially be very complex. The authors multiplied the near-surface measurements with a coefficient 0.85 (please provide proper referencing for this), which I am assuming corresponds to a fully developed boundary layer that obeys the law of the wall. However, the measurements are done in the vicinity of a bridge (actually they included the effect of the bridge by measuring after the flow passed the bridge by changing measuring locations during ebb and flood – in line 109, what does “face the flow direction during measurements” imply?), where local flow accelerations may take place and/or local variations on the bed level may be present with the development of scour holes. In addition, the interactions of fresh and salt water are completely neglected and it is assumed that there is no stratification or mixing that could affect the law of the wall. Finally, by estimating the flow discharge with this method, the (limited time period of) tidal reversal cannot be properly taken into account. As a result, the calculation of the flow discharge is questionable; however, it is used to deduce one of the main findings of the paper: the fact that the net transport of plastics is higher than the net discharge. Moreover, the way Equation (4) is written, implies that it doesn’t calculate the discharge of the whole cross section. In line 133 you mention that w_i=15 m (and W=298 m) and you only have 5 such widths. So, by summing these five areas, you estimate a partial discharge of the river. This is not necessarily a problem, but you relate this to the plastic transport, F, in Equation (1), which extrapolates the measured plastics transport from each width w_i to the whole river width W. By measuring so close to the bridge, it is expected that the bridge piers will induce a high variation in the flow velocities and the plastic concentration on the water surface across the river cross-section. Please clarify how these variables are connected.

Some other comments in order of appearance:

Lines 142-143: There is no justification about this assumption for the categories “Multilayer” and “Other plastic” and the choices seem rather arbitrary.

Lines 168-170: I agree with the authors that it’s less uncertain discussing flow velocities that are directly measured instead of the calculated flow discharges; however, a large part of the analysis is still done based on water discharges and water volumes. How reliable are your conclusions then?

Line 201: In the way that you defined the plastic transport (either items per hour in Equation 1 or mass per day in Equation 2), how can you get volume of plastic transport by integration?

Line 218: Why only higher net transport of plastics and not lower?

Line 224: Can you please clarify “with both higher peaks in flow velocity during the ebb than flood phase of the tidal cycles”

Lines 278-279: What are the three values and how is this related to the next sentence and the median mass of items?

Lines 337 and 404: As a reader, a paper that is under review does not provide strong support for an argument.

Lines 356-357: Is this reference at the tidal part of the river? If not, how is it related to your study?

Lines 392-394: This is a very vague and highly speculative sentence.